The AC universal motor has been used for a long time as the most widely used motor in small domestic appliances and electric devices powered by the single-phase mains. Although it has a number of drawbacks, such as a short lifetime, poor quality of operation, and loud noise, it is widely used because of its low cost.
For some time, solutions with electronically commutated motors have been proposed, which, however, have not come into use due to their higher cost price. The drawback of a higher cost price is caused by the required power electronics, which converts a DC source into a three-phase current system for which it requires a plurality of power conductors (generally six). Moreover, a quite substantial number of additional components is required in that a further power-electronic circuit (generally an up-converter) is required so as to keep the overshoot of the mains input current below legally required limit values. This circuit comprises further power-electronic components such as an expensive electrolytic capacitor. This part of the circuit covers approximately one third of the overall costs.
A known method of powering asynchronous motors without energy stores uses a direct changer (Sxc3xa4mann, EP 0 489 970, FIG. 9). In this method, however, the output frequency of the static inverter is limited to approximately ⅓ of the mains frequency. The method is thus not suitable for drives with rotational speeds of more than 950 min-1.
A further method shown in FIG. 5 of the drawing and described in xe2x80x9cUndeland, Mohan, Robbins: Power Electronics: Converters applications and design, Wiley and Sons, 1989, pp. 415 etc.xe2x80x9d uses an up-converter for supplying a high, approximately constant intermediate circuit voltage which can be supplied to a static inverter. Simultaneously, the mains input current is controlled in such a way that a substantially sinusoidal current is obtained. This circuit also comprises an energy store (21, usually an electrolytic capacitor), a semiconductor switch (17), a power choke (19) and a high-frequency diode (18). This method is universally usable but has the drawback of higher costs.
A method is known from DE 19729705 in which the intermediate circuit capacitor and a special phase switch of the static inverter achieves an effect which is seemingly similar to the effect achieved by the present invention. The associated circuit arrangement is shown in FIG. 7. In this method, it is, however, impossible to reduce the power arbitrarily because then this phase switch takes place when the intermediate circuit capacitor has not been discharged yet. This results in unacceptably high currents in the static inverter, except in high-speed motors with a sufficiently high spread. Consequently, the method is not suitable for low speeds and for drives whose power is adjustable within wide ranges.
In a further method known from EP 0 711 470 and shown in FIG. 6, the phase switching as described in DE 19729705 is dispensed with and, instead, a frequency modulation is utilized so as to operate the drive suitably. However, mains current oscillations, which are unwanted in the mains zero crossing, may then still occur, causing the harmonic limits to be exceeded at average power. Their use is limited to high-speed drives having a power which must not decrease below a minimum value during operation.
Further methods are known in which it is achieved by means of a special arrangement of diodes and storage capacitors (for example, xe2x80x9cValley-fillxe2x80x9d circuit shown in FIG. 8) that, in spite of a storage capacitor in the intermediate circuit, there flows a current during a comparatively large part of the mains period. These circuits mainly influence the angle of current flow in the mains. However, it is not possible to reduce the mains current harmonic content sufficiently enough to allow use of drives having an average and a high power.
The drive system according to the invention comprises a rectifier, a high-frequency filter, a static inverter and an induction motor, is driven without energy stores and fed by the single-phase mains, in which the use of an envelope generator with a mains-synchronous, preferably sinusoidal signal causes a preferably sinusoidal envelope to be sign-superimposed on the static inverter output signal, thus preventing inadmissible current overshoots in the single-phase mains. Possible fluctuations of the intermediate circuit voltage are thereby compensated.
In further embodiments of the invention, the advantageous operating frequency of the static inverter is adjusted automatically by means of a PLL control circuit in that the phase shift between the intermediate circuit current and the envelope signal is evaluated.
In a further embodiment of the invention, the advantageous operating frequency is gained by adding a given frequency difference to the rotational speed of the motor.
In further embodiments of the invention, the envelope directly represents a projection of the variation of the voltage of the single-phase mains.
The present invention relates to a method of advantageously driving an induction or asynchronous motor in its overall speed range from the single-phase mains so that the harmonic content of the mains input current of the circuit remains below the admissible upper limits. Moreover, the drive can be arbitrarily adjusted in power. It is also possible to realize generator operation by means of an active mains rectifier. This provides the possibility of manufacturing electronically commutated speed-controlled drives at low cost for substantially any speed range and substantially any application. A principal limitation of the power range, as in all the other methods, is thereby eliminated. Drives up to the typical power limit of 3 kW of the single-phase mains can thus be realized.